Atomic force microscopes (AFM) have been known which use a cantilever to measure the shape of a sample. The AFM is expected as a technique for observing, for example, the nano-level-function of biologic molecules. FM (Frequency Modulation)-AFM, a type of AFM, is known to be able to be used not only in a contact mode but also in a noncontact mode. The FM-AFM is also known to provide high-resolution images.
The FM-AFM generally self-excitedly oscillates a cantilever at a resonant frequency and senses the resonant frequency shift caused by the interaction between the cantilever and a sample to obtain information on the sample. A conventional typical FM-AFM will be described below.
FIG. 1 is a block diagram showing the conventional FM-AFM. In FIG. 1, a cantilever 1 has a probe at its leading end. The cantilever 101 is placed so that the probe is in proximity to a sample on a sample stage 103. A displacement sensor 105 is generally based on an optical lever and monitors the displacement of the cantilever 101.
A detection signal from the displacement sensor 105 is differentiated by a differentiator 107. The resultant signal is appropriately gained by an amplifier 109. The resultant driving signal for the cantilever 101 is then supplied to an actuator 111, which then drives the cantilever 101. The actuator 111 is, for example, a piezo element. No other external driving signals are present.
Here, the cantilever 101 is thermally fluctuated. The amplitudes thermally induced around the resonant frequency are larger than those at the other frequencies. This fluctuation appears in the detection signal from the sensor 105 (proportional to the displacement of the cantilever). The detection signal is differentiated and gained, and then supplied as the driving signal. Consequently, the magnitude of the driving signal is increased in the vicinity of the resonant frequency. The cantilever 101 oscillates in the vicinity of the resonant frequency more intensely than before the driving signal is provided. The oscillation is further reflected in the driving signal. This sharply increases the oscillation amplitude in the vicinity of the resonant frequency (increased Q value). The cantilever 101 thus oscillates self-excitedly.
The interaction between the probe of the self-excitedly oscillating cantilever 101 and the sample (they need not necessarily contact) apparently changes the spring constant of the cantilever 101 from the original value owing to the interaction force gradient between the probe and the sample. This shifts (changes) the resonant frequency of the cantilever 101. Thus, detecting the resonant frequency shift enables information on the sample shape to be obtained.
In FIG. 1, the resonant frequency shift is detected by a resonant frequency shift detecting circuit 121 on the basis of a detection signal from the displacement sensor 105. The resonant frequency shift detecting circuit 121 is conventionally usually composed of a phase locked loop (PLL) circuit.
A feedback circuit 123 generates a feedback signal for maintaining a constant resonant frequency shift, on the basis of the resonant frequency shift detected by the resonant frequency shift detecting circuit 121. The feedback signal is supplied to a scan control section 125.
The scan control section 125 controls a scanner 127 so that the sample stage 103 is scanned in an X direction, a Y direction, and a Z direction. The scanner 127 is composed of, for example, a piezo element. In accordance with instructions from a superordinate control section, the scan control section 125 drives the scanner 127 in the X and Y directions. At the same time, the scan control section 125 drives the scanner 127 in the Z direction so as to maintain a constant resonant frequency shift, on the basis of the feedback signal from the feedback circuit 123.
The feedback signal corresponds to the resonant frequency shift. The shift amount of the resonant frequency increases or decreases in accordance with the interaction force between the cantilever 101 and the sample. That is, the shift amount varies depending on the distance between the cantilever 101 and the sample. Therefore, the topographs of the sample can be measured on the basis of the feedback signal.
The FM-AFM is configured to detect the resonant frequency shift caused by the interaction between the self-excitedly oscillating cantilever 101 and the sample as described above. The resonant frequency shift occurs even if the probe and the sample are not in contact with each other. Specifically, the interaction is an attractive force between the probe and the sample, which reduces the resonant frequency. Thus, the FM-AFM is used as a noncontact AFM.
Further, if the probe contacts the sample, the interaction is repulsion which increases the resonant frequency. The resultant resonant frequency shift is detected. This resonant frequency shift is more sensitive to the interaction between the probe and the sample than a variation in amplitude. Utilizing this, the FM-AFM can offer a higher resolution than ordinary AC mode AFMs (tapping mode AFMs) which use amplitude variations.
The conventional FM-AFM is disclosed in, for example, Japanese Patent Laid-Open No. 2004-226237.
The conventional general FM-AFM uses a PLL circuit to detect the resonant frequency shift as described above. The PLL circuit is used in order to sensitively detect slight variations in resonant frequency.
However, the PLL circuit detects the resonant frequency shift on the basis of lever oscillation over a long time corresponding to a plurality of cycles. The PLL circuit thus has too low a detection speed, preventing the FM-AFM from achieving high-speed imaging. For example, observing the functional dynamics of biologic molecules requires observations in a short time. However, it is difficult to meet the higher-speed requirement as far as the PLL circuit is used.
The PLL circuit is also disadvantageous in terms of sensitivity as described below. The magnitude of the interaction between the probe and sample increases and decreases even during one cycle of cantilever oscillation. The interaction weakens when the probe is apart from the sample. However, the PLL circuit detects the average resonant frequency shift over a plurality of cycles of cantilever oscillation. That is, the PLL circuit detects the average resonant frequency shift over a long time including the periods when the interaction is weak. This is a factor reducing the sensitivity of detection of the resonant frequency shift.